1. Field of the Invention
This invention relates generally to blow molding, and is more particularly directed to the blow molding of completely finished, biaxially oriented hollow containers.
2. Description of the Prior Art
It is known in the prior art that the biaxial blow molding of hollow containers, such as bottles, from synthetic resins results in an article of improved physical properties, such as strength and transparency. Generally, in this blow molding technique, a cooled tubular preform or parison is heated to a suitable orientation temperature, stretched along its longitudinal direction to axially orient the material and then blown within a mold cavity.
In the blow molding of hollow containers, one particularly difficult problem is to produce large containers with standard-size neck and thread areas. In this situation, the cross-sectional area of the material in the neck is greater than in the remainder of the blown container. Accordingly, if a small parison is used to provide a small-diameter neck and threaded area, the walls of the remainder of the parison will be thinned unduly upon expansion during the blow molding process and detrimentally affect the strength of the container. Conversely, if a parison is used which has sufficient material to form walls of a relatively large container, the diameter of the neck and thread area will be too large, resulting in additional material being used unnecessarily in the formation of the container and possibly producing flash upon closure of the mold which would require additional processing to finish the container and thus add to the cost of production.
Certain physical characteristics of a container molded of biaxially oriented material, such as strength and clarity, are related to the axial draw ratio possible with a given parison size. The axial draw ratio is determined by the unstretched length of the parison and the length after stretching. These lengths are determined by the desired container size and weight which, in conjunction with the need to provide a standard-size neck and thread portion, define the parison size. Thus, there are techniques in the prior art for blow molding biaxially oriented containers in which the achievable axial draw ratios are limited and the maximum size of the containers which can be formed is restricted.
Biaxially oriented containers have been made by clamping a portion of the parison within neck-forming cavities before stretching of the parison. Techniques are also known in which preforms with finished neck portions molded in a first, preform molding procedure are then stretched and blow molded. The resulting containers have oriented material in the body, but the finished neck does not have oriented material. The neck region, therefore, lacks the clarity and strength that is partially obtainable in a container having at least axially-oriented material. To overcome this problem, processes have been developed which provide orientation of the material in the container neck region. Examples of such processes are disclosed in U.S. Pat. No. 3,651,186, issued to J. N. Hall and U.S. Pat. Nos. 4,065,355 and 4,116,607, issued to R. W. LeGrand. In U.S. Pat. No. 4,108,937, issued to Martineu et al., a cylindrical preform is stretched prior to blow molding of the container, resulting in orientation of the material in the neck region, and the material in this region is heated to a higher temperature to permit greater deformation. However, unlike the LeGrand process, the Martineu et al. process does not provide for the formation of closure attachment means on the neck surface such as threads nor is there a precise calibration of the neck opening inside diameter. Instead, in Martineu et al. the molded container is removed and a separate finishing operation is required to cut the upper portion of the container to produce the final form of the finished container.
In the LeGrand patents, a technique is disclosed for parison stretching and container neck forming in which oriented material is forced by a crammer sleeve into the thread forming area of the mold. The outside diameter of the crammer sleeve is substantially the same size as the outside diameter of the finished neck as measured at the base of the threads, and the outer diameter of the blow pin establishes the diameter of the opening in the finished neck. Since the parison must fit over the crammer sleeve, the inside diameter of the parison is accordingly fixed. This size constraint and the requirement of a standard-size neck fixes the ratio of the inside-to-outside diameters of the parison. The neck size is a critical factor since the parison's outside diameter in the container neck region, after stretching, must fit within the opening provided by the mold neck inserts to prevent flash formation. With the parison diameters thus fixed and the desired container's size and weight parameters determining the length of the parison, the axial draw ratio achieveable is also fixed for a given size container, mandrel and neck thread mold inserts. This technique, therefore, limits the flexibility in selection of the axial draw ratio.
A related problem in the prior art technique of forming containers of biaxially oriented material having finished necks is the provision of an adequate quantity of oriented material to form closure attachment means such as threads. Thus, in a technique such as disclosed in the aforesaid LeGrand patents in which the parison is axially stretched onto a calibrating blow pin and the mold sections closed to size the outer diameter of the container neck, there may be insufficient axially-oriented material available below the crammer sleeve to provide well-defined shapes for all the threads. Merely increasing the travel distance of the crammer to increase the amount of material disposed below the crammer is not a completely satisfactory solution since the crammer would then be required to pass through a larger quantity of axially-oriented material, which may adversely affect the capability of the crammer to form a smooth, cleanly-severed neck end surface. An excessive amount of oriented material below the crammer may also adversely affect the distribution of the material along the length of the container neck such that the threads and the lower end of the neck are improperly formed.